1. Field of the Invention
The present invention relates to a method for performing a handover among base stations in a broadband mobile communication system while guaranteeing the operability of a mobile station. More particularly, the present invention relates to a system and method for performing a handover by considering Quality of Service (QoS) in a broadband mobile communication system, in which the QoS, as well as the strength of a received signal between a mobile station (MS) and a base station, is taken into account by using neighboring base station information when performing a handover in a multi-cell boundary area.
2. Description of the Related Art
Generally, research has been actively conducted in order to provide users with services including various Quality of Services (QoS) allowing data transmission at a speed of nearly 100 Mbps in a future communication system, i.e., the 4th generation (4G) communication system. In particular, research has been actively conducted in the area of supporting a high-speed service while guaranteeing mobility and QoS in a Local Area Network (LAN) system, a Metropolitan Area Network (MAN) system and a Broadband Wireless Access communication system. A representative communication system regarding the research corresponds to an Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system and IEEE 802.16e communication system.
The IEEE 802.16a communication system and the IEEE 802.16e communication system apply an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme in order to support a broadband transmission network to a physical channel of a wireless MAN system. Moreover, the IEEE 802.16a communication system considers only both a single cell structure and a fixed state of a Mobile Station (MS), i.e., a state where the mobility of the MS is not taken into account. On the other hand, the IEEE 802.16e communication system considers the mobility of the MS of the IEEE 802.16a communication system, and the MS is referred to as an MS having a mobility.
A structure of the IEEE 802.16e communication system in a broadband mobile communication system will now be described with reference to FIG. 1.
FIG. 1 is an example that schematically illustrates a conception of a handover of multi cells in a broadband mobile communication system. Referring to FIG. 1, the IEEE 802.16e communication system has a multi-cell structure, and provides a given MS 100 with service in a corresponding cell area where each of base stations (102, 104 and 106) is located. Signal transmission/reception between each of the base stations (102, 104 and 106) and the MS 100 is accomplished by using the OFDM/OFDMA scheme.
However, the MS is located in a boundary area (i.e. handover area) of cells controlled by all of base station 102, base station 104, and base station 106, respectively. Accordingly, it is possible to support mobility of the MS 100 only when a handover of the MS 100 is supported.
Here, the handover refers to a process regarding communication according to a movement of a given MS from the area of a base station to the area of a new base station or to a new antenna permitting area, in another words, to a new traffic channel. It is important to keep a mobile call successful and a quality of voice information at a certain minimum predetermined level without deterioration during the handover process. It can be said that handover is a procedure of setting a new communication path between an optimal base station and the MS by efficiently recognizing changes in the communication environment. That is, the handover corresponds to a procedure of establishing a communication path to a neighboring base station whose communication environment is superior to a communication environment of a current serving base station when an MS is located in a cell boundary area.
FIG. 1 shows that, while an MS 100 is in a cell area controlled by the base station 102 and performs communication with the base station 102 through a connection setting therewith, the MS 100 moves to a cell area controlled by the base station 104 or the base station 106 and performs a handover in order to communicate again. In this case, when the MS 100 is located in a cell boundary area where a handover is required, the MS 100 determines a handover in consideration of strengths of the signals that the base stations 104 and 106 transmit to the MS 100. That is, as shown in FIG. 1, when a strength of the received signal from the base station 104 is higher than a strength of the received signal from the base station 106, the MS performs a handover with the base station 104.
FIG. 2 shows a relation among the MS 100 located in a cell boundary area and the strengths of the received signals from the base stations 102, 104, and 106, and a distance between the MS 100 and the base stations 102, 104, and 106. FIG. 2 is a graphically illustrates a signal strength between an MS and a base station during a handover in a broadband mobile communication system.
Referring to FIG. 2, when a reception signal strength between an MS 100 and a Radio Access Station RAS1 goes below a certain level (T_DROP), a handover is generally performed. As shown in FIG. 2, it is understood that based on the signal strength, a handover is going to be performed to hand the MS 100 over to the RAS2.
As described above, if the MS performs a new connection setup whenever the MS changes a connection base station according to an optimal reception signal strength, the MS will attempt a new connection at every change of the base station. In order to easily establish a connection with a new base station as the MS moves, in a conventional broadband wireless access communication system, a current base station connected to the MS provides the MS with information about neighboring base stations, to which the MS on the move is likely to be newly connected thereto, through a neighboring base station information advertisement message.
Accordingly, it is preferable that the MS should be first aware of information about neighboring cells, i.e., neighbor base stations, in order to move to a neighboring cell and perform a handover thereto. Therefore, the MS cyclically receives a neighbor base station advertisement message from a serving base station which currently provides a service to the MS and collects information about the neighbor base stations.
FIG. 3 illustrates an example of a situation where only a signal strength between an MS and a base station is considered during a handover in a conventional broadband mobile communication system. Referring to FIG. 3, when an MS 300 is located in an area where a handover is required, the MS 300 receives information about neighboring base stations 302, 304 and 306 from a serving base station (not shown) with which the MS 300 currently communicates, and receives a signal from each of the neighbor base stations 302, 304 and 306 to thereby perform a handover based on the strength of the received signal from the base stations 302, 304 and 306.
Still referring to the example shown FIG. 3, the base station 302 is communicating with MSs 31, 32 and 33, respectively, and a utilization of the base station 302 corresponds to 40% of capacity. The base station 304 is communicating with MSs 34, 35, 36, 37, 38 and 39, respectively, and a utilization of the base station 304 corresponds to 90%. The base station 306 is communicating with an MS 30 and a utilization of the base station 306 corresponds to 10%. In this case, when the MS 300 performs a handover to a base station 304, which has transmitted an optimal strength of the received signal, by considering only the strength of the received signal, an optimal QoS cannot be guaranteed for the handed-over MS 300 because the utilization of the base station 304 corresponds to 90% of capacity.
In view of the example shown in FIG. 3, when an MS Neighbor Advertisement (MOB_NBR_ADV) message includes only information about the neighboring base station is transmitted to the MS, and the MS performs a handover by searching a base station, which transmits an optimal received signal, on the basis of the neighbor base station information. However, when conventionally performing a handover in a multi-cell boundary area such as in the example in FIG. 3, there occurs a problem regarding an inability to guarantee a certain QoS.